Bigby's Hand spells are blunt combat spells, while Arcane Trickster is supposed to be more about subtlety and (well...) "tricks"
But but but... Arcane Tricksters
can learn Bigby's spells!
They can learn up to 4 non-Enchantment/non-Illusion spells!
(edit) err... now that I think about it, there is just a small problem with that... Bigby's Hand is a 5th level spell, and Arcane Tricksters cast spells only up to 4th level! So no Bigby's Hand for Arcane Tricksters
but the
could learn it if it wasn't for the high level